Maliks Muwatta Book 50, Hadith Number 15.

Section : Washing with Water for a Fever.

Yahya related to me from Malik from Hisham ibn Urwa from Fatima bint al-Mundhir that whenever a woman who had a fever, was brought to Asma bint Abi Bakr, she made dua for her and took water and poured it inside her collar. She said, “The Messenger of Allah, may Allah bless him and grant him peace, ordered us to cool it with water.”